The term thermal printing covers two main technology areas. In thermal transfer printing of textiles, a donor sheet is coated with a pattern of one or more dyes, contacted with the fabric to be printed, and heat is uniformly administered, sometimes with concomitant application of a vacuum. The transfer process has been much studied, and it is generally accepted that the dyes are transferred by sublimation in the vapor phase. Pertinent references include: Bent, C. J. J. Soc. Dyers Colour. 1969, 85, 606; Griffiths, J.; Jones, F. Ibid. 1977, 93, 176; Aihara J. Am. Dyest. Rep. 1975, 64, 46; Vellins, C. E. In The Chemistry of Synthetic Dyes; Venkataraman, K., Ed.; Academic Press: New York, 1978; Vol. 8, p 191.
The other area of thermal printing is thermal imaging, where heat is applied in an image-wise fashion to a donor sheet in contact with a suitable receptor sheet to form a colored image on the receptor. In one embodiment, termed thermal mass transfer printing, as described for instance in U.S. Pat. No. 3,898,086, the donor is a colorant dispersed in a wax-containing coating. On the application of heat the construction melts or is softened, and a portion of the colored donor coating transfers to the receptor. Despite problems with transparency, pigments are generally the colorants of choice in order to provide sufficient light fastness of the colored image on the receptor.
Another embodiment is termed variously thermal transfer imaging or recording, or dye diffusion thermal transfer. In this case, the donor sheet comprises a dye in a binder. On image-wise application of heat, the dye, but not the binder, is transferred to the receptor sheet. A recent review has described the transfer mechanism as a "melt state" diffusion process quite distinct from the sublimation attending textile printing (Gregory, P. Chem. Brit. 1989, 25, 47). This same review emphasizes the great difficulty of developing dyes suitable for diffusive thermal transfer. With regard to the available conventional dyes, it was stated that ". . . It is significant that of the one million or so dyes available in the world, none of them were fully satisfactory . . . ". Among the failings of these dyes are inadequate light and heat fastness of the image and insufficient solubility of the dyes for coating in the donor sheet. As has been noted previously, light fastness is also a problem in mass transfer imaging systems. In fact, achieving adequate light fastness is probably the single most important challenge in these constructions. In large measure this is the result of the diffusive thermal transfer dye image being a surface coating a few microns thick. The dye is thus readily susceptible to degradation by photo-oxidation. In contrast, textile fibers, which are 100 times thicker, are uniformly dyed throughout their depth, so that fade in the first few microns at the surface is of little practical importance. In consequence, it is common to find that dyes showing good light fastness in textile printing exhibit very poor photostability in the diffusive thermal imaging (see, for example U.S. Pat. No. 4,808,568). There remains, therefore, a strong need for improved dyes for this latter application.
Metal-azo dyes, having one dye to one metal, are known in the art. The following references discuss the preparation of these materials: Drew, H. D. K.; Fairbairn, R. E. J. Chem. Soc. 1939, 823-835; Beech, W. F.; Drew, H. D. K. J. Chem. Soc. 1940, 608-612; Steiner, E.; Mayer, C.; Schetty, G. Helv. Chim. Acta. 1976, 59, 364-376; U.S. Pat. Nos. 4,012,369; 4,123,429; and 4,265,811. Metal-azo 1:1 complexes are predominantly used in two applications, color photography and the dyeing of textiles.
The following are examples of the use of 1:1 complexes in the photographic field: U.S. Pat. Nos. 3,453,107; 3,551,406; 3,544,545; 3,563,739; 3,597,200; 3,705,184; 3,752,836; 3,970,616; 4,150,018; 4,562,139; and 4,767,698. One embodiment of color photography, termed color diffusion transfer photography, employs non-diffusible, dye releasing compounds which are 1:1 complexes. In this embodiment, a ballasted carrier moiety, capable of releasing the dye as a function of development of the silver halide emulsion layer under alkaline conditions, is incorporated into the metal-complex. The 1:1 complex then diffuses through gelatin to a receiving element. The constructions require the presence of a silver halide emulsion layer and a "ballasting" group covalently attached to the metal-complex. Chemistry is required in order to create a diffusible moiety.
The following references are to 1:1 complexes used in textile dyeing: U.S. Pat. Nos. 3,878,158; 4,218,367; 4,617,382; and European Pat. 144776.
For the most part, the 1:1 complexes discussed in the two preceding paragraphs are chromium(III) complexes containing a tridentate azo dye, a monoanionic bidentate ligand (e.g. acetylacetonate), and a monofunctional monodentate ligand. The monofunctional ligand is generally H.sub.2 O, although, examples where the ligand is pyridine, ammonia, or ethanolamine are also described.
Metal complexes containing polymerizable functionality are known. The metal vinylpyridines complexes are representative members of this class. Selected references to metal vinylpyridine complexes are: U.S. Pat. No. 3,287,455 and Agnew, N. H.; Collin, R. J.; Larkworthy, L. F. J. Chem. Soc., Dalton Trans. 1974, 272-277. For the most part, the color of these materials is due to weakly absorbing metal-centered ligand field transitions. Some cobalt(II) derivatives are reported to be deep blue (Agnew, N. H.; Larkworthy, L. F. J. Chem. Soc. 1965, 4669-71). The color in these systems is also due to metal-centered transitions, however, in a distorted tetrahedral environment. Generally, the extinction coefficients of visible wavelength transitions in these metal complexes are less than 1000 M.sup.-1 cm.sup.-1 which make them, in general, unsuitable as dyes or colorants.
Many transition metal complexes with vinylpyridine as a ligand are unstable. Some of these complexes are quite labile in solution, exhibiting the following equilibrium: EQU M(ligand).sub.x (vinylpyridine).sub.n .revreaction.M(ligand).sub.x (vinylpyridine).sub.n-1 +vinylpyridine
Additionally, transition metals, such as copper(II) and ruthenium(III), may initiate the polymerization of vinylpyridine (e.g., Tazuke, S.; Okamura, S. J. Polym. Sci.: Part A-1 1966, 4, 141-57 and Norton, K. A., Jr.; Hurst, J. K. J. Am. Chem. Soc. 1978, 100, 7237-42), although some stable complexes of copper(II) and vinylpyridine have been reported (Laing, M.; Horsfield, E. J. Chem. Soc., Chem. Commun. 1968, 735).
These examples demonstrate the complexity of predicting the stability of metal complexes containing polymerizable groups. There are still other examples where the vinyl group undergoes a cyclometallation reaction with the metal (Newkome, G. R; Theriot, K. J.; Cheskin, B. K.; Evans, D. W.; Baker, G. R. Organometallics 1990, 9, 1375-9.
There is very little reference to the use of metal-azo dyes in thermal printing art. A review on transfer printing (Datye, K. V.; Vaidya, A. A. Chemical Processing of Synthetic Fibers and Blends; John Wiley & Sons: 1984, p 407) states: "Acid and metal-complex dyes which are commonly used for dyeing nylon are unsuitable for heat-transfer printing because these dyes have high melting points and low vapor pressures and hence, do not get vaporized and transferred below 200.degree. C. However, the recently developed Dew Print.TM. machine enables wet-transfer printing of the acid and metal-complex dyes on nylon." The wet-transfer-process dyes of the above reference require the presence of water solubilizing groups such as sulfo and carboxy, and the dyes are generally charged. This process involves the dissolution of the dye in water and transfer to the substrate. Further details of this process are given in U.S. Pat. No. 4,155,707.
Metal-azo dyes have been used in mass transfer printing. In Japanese Pat. No. 62021594-A, it is stated that "the ink layer is completely transferred to plain paper when the transfer recorder is peeled from plain paper"--a clear indication that both the binder and the colorant are transferred. Moreover, the binders used in the practical examples are all low molecular weight (less than 2000 Daltons), except for the control which was demonstrated to not transfer efficiently. The colorants used were high melting pigments, some of which were calcium or sodium salts of azo dyes. These salts are ionic in nature and are generally not soluble in organic solvents. In a related case (Japanese Pat. No. 62021593-A) the process being discussed is also mass transfer, however, the colorants were "oil soluble". Some of these oil soluble dyes were metal-azo dyes, wherein the structures were not explicitly disclosed. The metal-azo dyes that could be identified were found to be negatively charged 2:1 (metal:azo) complexes. The solubility characteristics of the dyes, for which structures were not available, indicate that they are probably 2:1 complexes, as well.
Other embodiments of mass transfer systems utilizing metal-azo dyes are discussed in U.S. Pat. Nos. 4,585,688, 4,664,670, and 4,784,905. Described in U.S. Pat. No. 4,585,688 is a transfer medium comprised of a heat-resistive support, a colorant layer containing a binder and a coloring agent (which may be a metal-azo dye) and a transferrable layer comprising a low molecular weight compound capable of containing a coloring agent and transferring an image to a paper receptor. In U.S. Pat. No. 4,664,670, a thermal transfer donor construction requiring the presence of a low melting, essentially colorless, non-polymeric, organic nitrogen-containing, impregnating reagent for the printing of textiles is disclosed. A thermosensitive image transfer recording medium comprised of a support material and a thermofusible ink layer is described in U.S. Pat. No. 4,784,905. The thermofusible ink layer contains a fine porous resin structure made of a resin containing: (1) a coloring agent (which may be a metal-azo or metal-azomethine dye), (2) a carrier material (for holding the coloring agent at normal temperatures and also for carrying the coloring agent out of the thermofusible ink layer for image formation upon application of heat), and (3) an image gradation control agent.
There are also several published patent applications (see, for example: Japanese Publ. Appl. Pat. Nos. 63-144,084, 60-002,398, and 59-078,893-A) which disclose the use of metallizable azo dyes in thermal transfer donor constructions. In these cases, the donor layer comprises an azo dye, capable of chelating to a metal, and a binder. The azo dye is thermally transferred to a receptor layer which contains a metal salt which can react with the azo dye. The generation of a metal-azo dye by this method has several potential drawbacks because (1) the colors of the azo dyes and the metallized dye are different, the resultant color will depend on the extent of metallization, (2) metallized dyes are generally much more resistant to light induced fade and therefore, if both azo dye and metallized-azo dye are present the color may change as a function of light exposure, (3) the chelation of the azo dye to a metal often involves the generation of acid which could have a deleterious effect on image stability. This problem can be overcome by addition of buffering agents, however, this further complicates the donor or the receptor formulation.